Understanding colloidal FeSx formation from iron phosphate precipitation sludge for optimal phosphorus recovery

https://doi.org/10.1016/j.jcis.2013.04.001Get rights and content

Highlights

  • Phosphate can be recovered from synthetic FePO4 via S2− addition.

  • Much improved settling of the colloidal FeSx particles was obtained at pH 4.

  • The isoelectric point of colloidal FeSx particles was found to be at pH 4.

  • An efficient method for P recovery from chemical iron-based sludge is demonstrated.

  • The method proposed is a step toward the sustainable use of P as a resource.

Abstract

The use of sulfide to form iron sulfide precipitates is an attractive option for separation and recovery of phosphorus and ferric iron from ferric phosphate sludge generated in wastewater treatment. The key factors affecting the simultaneous generation and separation of iron sulfide precipitates and phosphate solution from ferric phosphate sludge have so far not been thoroughly investigated. This study therefore focuses on the recovery of phosphorus from synthetic sludge by controlled sulfide addition under different operating conditions. The factors that affect the phosphorus recovery, as well as the optimal process conditions to achieve an effective solid–liquid separation, were investigated. The separation of the FeSx particles is a significant challenge due to the colloidal nature of the particles formed. Faster separation and higher phosphorus recovery was achieved when operating at pH 4 with dosing times of at least 1 h. At this pH, phosphorus recovery of 70 ± 6% was reached at the stoichiometric S/Fe molar ratio of 1.5, increasing to over 90% recovery at a S/Fe molar ratio of 2.5. Zeta potential results confirmed the colloidal nature of the iron sulfide precipitate, with the isoelectric point around pH 4, explaining the fast separation of the FeSx particles at this pH.

Introduction

Phosphorus is one of the most important nutrients required for plant growth. Nearly all the phosphorus used in agriculture comes from phosphate rock mines. However, it is estimated that the phosphate left from mining will last at most about 50–100 years [1]. This has been a driving force toward finding renewable sources. The utilization of wastewater as a nutrient source has increased in the last years [2], with digested sewage sludge being a major target as it contains 2–3% of elemental phosphorus.

Methods for phosphorus recovery from wastewater sources such as struvite (magnesium–ammonium–phosphate, MAP) formation [3], [4], [5], [6] and the recovery of phosphorus and aluminium from sewage sludge ash by sequential elution with acidic and alkaline solutions (SESAL-Phos-recovery process) [7] have been developed. Similarly, methods for coagulant recovery from ferric (Fe3+) and aluminium (Al3+) precipitation sludge have been proposed such as selective removal using ion exchange [8], [9] and acidic extraction of coagulant [10]. However, limited work has been reported on simultaneous recovery of phosphate and ferric iron.

The advanced treatment of secondary effluent, e.g. for the production of purified recycled water, often includes a chemical precipitation process, in which coagulants such as FeCl3 are added in order to remove suspended solids, colloids, and phosphate. The sludge formed in this process contains mainly ferric oxy-hydroxides and phosphate, commonly termed ferric phosphate sludge. The simultaneous recovery of phosphate and ferric from ferric phosphate sludge using H2S was first proposed two decades ago [11]. However, in this initial study, no information was provided as to the factors affecting the phosphorus recovery process, i.e. pH, separation method, and performance data such as phosphate and iron recovery efficiencies. In this process, sulfide (H2S, HS, and S2−) reacts chemically with iron phosphate, releasing phosphate into solution while precipitating iron sulfide and elemental sulfur [12]:2FePO4(s)+3H2S2FeS(s)+S(s)0+2H2PO4-+2H+

Wei and Osseo-Asare [13] described the formation of iron sulfides and elemental sulfur from the reaction of Fe3+ and HS in aqueous solution via the following reactions:2Fe3++HS-2Fe2++S0+H+Fe2++HS-FeS(s)+H+FeS(s)+S(s)0FeS2(s)

The formation of the different iron sulfide species strongly depends on the pH, as shown by Wei and Osseo-Asare, who found that in the pH range 3.6–5.7, mostly pyrite (FeS2, Eq. (4)) was produced. They attributed this to the high concentration of the precursors mackinawite (iron monosulfide, Eq. (3)) and elemental sulfur. Additionally, using Energy-Dispersive X-Ray Spectroscopy [EDS] analysis, a strong peak was seen at pH 4 for iron monosulfide and elemental sulfur. This agrees with the Fe/S Pourbaix diagram [14], where FeS exists as a stable compound at pH > 4 and redox potential lower than −0.001 V. Likewise, iron monosulfide formation was seen at pH up to 7.2. However, at pH  7.2, FeS formation could not compete successfully with γ-FeOOH formation, explaining the absence of pyrite as a reaction product [13], [15], [16], [17]. The pH dependence on the stability of FeSx and soluble sulfur species was also observed by others [18], [19], [20].

In a more recent study, Kato et al. [12] evaluated phosphate recovery methods using sulfide based on the process proposed by Ripl et al. [11]. The focus was on maximizing phosphate recovery efficiencies and to validate the method itself. Experiments with pre-coagulated sewage sludge and FePO4 synthetic sludge were therein performed in the pH range of 5.3–6.9 and at S/Fe molar ratios between 1.0 and 2.0, leading to a phosphorus recovery up to 44% from the pre-coagulated sludge and 92.8% from the synthetic FePO4 after centrifugation.

Even though the proposed phosphorus recovery process via iron sulfide generation has been further studied by others [12], [21], [22], the selected solid–liquid separation processes, e.g. precipitation and centrifugation [11], [12] present significant challenges for the recovery efficiency and operating costs due to the poor settling characteristics of FeSx particles. When in contact with sulfide, phosphate is released into the solution and a black material is formed. The composition of this black material has been found to be a mixture of FeS and FeS2[13]. Its exact composition depends on the pH. Fe(HS)+ was also found in solution in the pH range of 3.6–7.8 [13], [19], [20], [23]. Despite the proven fast release of phosphate into solution, the separation of the formed suspension has been a challenge. At neutral pH or higher, the precipitated black material remains in suspension for more than a week and passes through a 0.22 μm filter, as preliminary tests in our lab have shown (data not shown).

Among the parameters that may affect the solid–liquid separation in this system, mixing time in the reaction vessel and dosing rate of sulfide have been previously identified as key factors for iron sulfide precipitation in different systems. Gutierrez et al. [24] demonstrated that the hydraulic retention time (HRT) of iron sulfide in sewers crucially impacts the formation of iron sulfide precipitates during primary settling, i.e. at shorter HRT a higher concentration of iron sulfide in the primary effluent was found. These findings suggest that mixing time in the reaction vessel is a crucial factor to enable effective separation of the formed iron sulfides.

Metal sulfide salts have low solubility. Thus, supersaturated solutions by interaction of aqueous sulfide and metal are expected [16]. Under these supersaturation conditions, fast nucleation is expected to limit crystal growth, thus generating poor settling characteristics of the formed metal sulfides [18], [23], [25]. At a microscopic level, the dosing rate and mixing speed are key factors in the nucleation process, i.e., slow dosing rates and mixing speed are expected to hinder nucleation and enhance instead particle growth and thus precipitation [26].

Even though considerable effort has been placed on improving the efficiency of this process, little systematic research has been carried out to determine the factors that affect phosphate recovery and separation of the formed FeS colloidal particles.

The aim of this research was to identify and characterize key factors that influence phosphate recovery and to determine the optimal process conditions to achieve an effective solid–liquid separation. In particular, the effects of pH, mixing time, sulfide dosing rate and settling time have been investigated in detail in this study. In order to elucidate the reaction chemistry and to understand the mechanisms of precipitation of the inorganic iron sulfide particles, most experiments in this work were carried out using synthetic FePO4 sludge, as a way to eliminate potential interference of organic solids, which are invariably present in ferric phosphate sludges obtained from wastewater treatment.

Section snippets

Preparation of synthetic sludge and solutions

In order to elucidate the stoichiometry of the iron phosphate reaction with sulfide and to determine the corresponding S/Fe ratio needed to achieve completion of the precipitation reaction, studies using synthetic iron phosphate suspensions were performed. A 0.1 M ferric phosphate suspension was prepared from FePO4*4H2O (97% pure from Sigma–Aldrich) to simulate the real sludge. Based on a typical Fe3+ concentration in real sludge, a 0.1 M suspension was prepared by mixing the salt in water

Reaction stoichiometry and phosphorus recovery

Based on the stoichiometry (Eq. (1)), it is expected that 1.5 moles of sulfide react with 1.0 mole of FePO4. This was confirmed after overdosing sulfide up to a S/Fe ratio of 2.5, where the S/Fe molar ratio reached in the solid phase was approximately 1.5 regardless of sulfide overdosing, thus indicating full reaction of Fe, according to reactions (2), (3), (4)(Fig. 2). The measurements reveal some missing sulfur from the liquid phase at higher S/Fe ratios, possibly due to some loss as H2S during

Conclusions

The aim of this research was to determine the optimal process conditions to achieve an effective solid–liquid separation and phosphate recovery from iron phosphate sludge via sulfide addition. After assessing the influence that pH, mixing time, and sulfide dosing rate, it was concluded that:

  • Effective phosphorus recovery from iron phosphate sludge can be achieved by sulfide addition, reaching 70 ± 6% recovery at a S/Fe stoichiometric molar ratio of 1.5 and increasing up to 92 ± 6% as the S/Fe molar

Acknowledgments

We thank Prof. Korneel Rabaey for his valuable input during the early stages of the Project, Mr. David Page for assisting with the particle size and zeta potential analysis and the AWMC Analytical Services Laboratory (ASL) for the ICP analyses. Elena Mejia Likosova thanks The University of Queensland for scholarship support. The Australian Research Council (LP100200122) together with the industry partners Seqwater and Veolia Water funded this work.

References (31)

  • D. Cordell et al.

    Global Environ. Change

    (2009)
  • F. Fischer et al.

    Bioresour. Technol.

    (2011)
  • J.D. Doyle et al.

    Water Res.

    (2002)
  • J. Keeley et al.

    Desalination

    (2012)
  • D. Petruzzelli et al.

    Water Res.

    (2000)
  • D. Wei et al.

    Colloids Surf., A

    (1996)
  • J. Horvath et al.

    Corros. Sci.

    (1964)
  • A.M. Jones et al.

    Geochim. Cosmochim. Acta

    (2009)
  • A.E. Lewis et al.

    Hydrometallurgy

    (2006)
  • D. Wei et al.

    Colloids Surf., A

    (1997)
  • A.E. Lewis

    Hydrometallurgy

    (2010)
  • D. Rickard

    Chem. Geol.

    (1989)
  • D. Firer et al.

    Sci. Total Environ.

    (2008)
  • N.G. Harmandas et al.

    J. Cryst. Growth

    (1996)
  • M.F.M. Bijmans et al.

    Water Res.

    (2009)
  • Cited by (28)

    • Enhancing phosphorus recovery from sewage sludge using anaerobic-based processes: Current status and perspectives

      2021, Bioresource Technology
      Citation Excerpt :

      The FeS and S in Eq. (5) are products of Eqs. (1) and (2). The result may be that the formation of FeS2 is not required more sulfide, in which the stoichiometric ratio of Fe/S is 1.5 (initially all Fe3+–P) or 1 (all Fe2+–P) for Fe–P release, but the subsequent solid–liquid separation and P recovery may be affected; it has been found that the FeS2 colloidal system is stable and hard to precipitate (Likosova et al., 2013). Sulfide is not initially existed in sewage sludge, but biologically reduced from sulfate (Costa et al., 2020).

    View all citing articles on Scopus
    View full text